Medical instruments for insertion into body cavities find widespread use. These are e.g. catheters, electrodes, sensors, imaging aids and guide wires that are inserted through trachea, blood vessels, urethra or other celoms or tissues.
Such instruments are required to have a high degree of smoothness to assure introduction thereof into a body cavity or via another medical instrument without causing trauma to tissue encountered during placement and manipulation.
The use of inserted medical instruments may produce friction and abrasive forces that apply to the surfaces of the medical device. It is desirable for the frictional resistance between the medical device being inserted and it's contact surface, be it another device or the contact surface within a patient, to be low. Relatively high friction between, for instance, a catheter and a guide wire may not only inhibit the guide wire from being inserted through the catheter smoothly, but can also inhibit the easy movement of the guide wire through the catheter; making it more difficult to carry out subtle indwelling operations at, for instance, the destined blood vessel site.
Catheters typically consist of plastic tubes which may have a single lumen or multiple lumens. Catheters may have balloons fastened along the tube to obstruct a vessel or to fix the catheters in a desired position. Catheters may also have ports at the distal end, side ports along part of the length, or other mechanical features needed to accomplish the particular device's mission. Catheters may consist of a continuous length of tubing, or may comprise two or more sections of tubing consisting of similar or dissimilar materials which are welded together in order to have different properties at different locations along the length of the device. Catheters may be tapered, both within a segment or by having segments of differing diameters. Typical material of which catheters are constructed include polyamides, polyurethanes, polyvinyls such as polyvinylchloride, polyesters, polyolefins, silicones, and others. Typical diameters range from less than one millimeter to more than 8 millimeters.
As typically encountered in inserting a catheter, at the predetermined site, the guide wire tip is inserted through a catheter up to its tip opening, the catheter with the guide wire is inserted into, e.g., a blood vessel percutaneously, and the catheter is further inserted through the vessel by using the guide wire as a leading and supporting guide. These operations produce friction and abrasive forces that apply to the surfaces of the medical device. It is desirable for the frictional resistance between the catheter inner surface and the guide wire to be low. Relatively high friction between the catheter and the guide wire not only prevents the guide wire from being inserted through the catheter, but also prevents the guide wire from being easily moved through the catheter, making it difficult to carry out subtle indwelling operations at the destined vessel site. Sometimes the guide wire cannot be withdrawn from the catheter, rendering the catheter lumen unusable despite the completion of the indwelling operation.
To avoid such problems, attempts have been made in the art to apply low frictional resistance materials such as Teflon® or silicone oil to the outer surface of guide wires. Application of silicone oil fails to retain lubricity because of immediate loss of silicone coatings. Frequent applications add to frictional resistance, also undesirably creating troubles as mentioned above.
There is thus the need for a catheter and guide wire having a lower frictional resistance surface which enables more subtle operation in a blood vessel and can be easily inserted and remain at the site where catheters are otherwise difficult to manage during placement.
Polyurethane coatings have been applied directly on metal surfaces. A reference in this respect is U.S. Pat. No. 4,876,126. However, commercial versions of this technology require thick layers (60-80 microns thick) in order to perform adequately. In practice, the thick layer extends continuously around the coated metal substrate. These layers have good cohesive forces and thus appear to be tightly bound on the metal surface, even though these layers do not necessarily have good adhesion to the metal surface. A disadvantage of such coatings is that because the polyurethane and other plastic layers are so thick, the metal diameter of the underlying wire must be correspondingly diminished. This is especially troublesome on the very fine wires such as those used in cardiovascular medicine (e.g. in coronary angioplasty) or in neurointerventional catheterization procedures.
Illustrative wire and coiled wire diameters range from 200 microns to more than 2 millimeters in diameter, the total length of the wire or coiled wire containing device is commonly in the range of 50 cm to 2.5 m, the coiled part making up anywhere from 1 cm to the full length of the wire depending on the desired mechanical properties such as flexibility, torsional stability etc. All or only part of the device may require a coating.
Particularly in respect of coiled wires, it is however not simple to substitute a coating for the polyurethane jacket, as the requirements for such a coating are inevitably stringent. The coating should have sufficient elasticity to bend with the coiled wire and it should have sufficient adhesion to the metal. Also, it should be sufficiently wear-resistant to withstand the process of insertion.
Reference is made to WO 91/19756, which discloses a method for providing a metal wire with a lubricious, hydrophilic topcoat, wherein a two-layered coating is applied. The teaching of this reference is particularly directed to a process which involves applying a first latex coating, then applying a second coating, and ensuring that the first coating is not cured before the second is applied, after which both coatings are simultaneously cured with heat. The document seeks to provide coatings of sufficiently strong adhesion. It does not address the aforementioned more advanced set of desired properties. As the aforementioned heat curing process is of essence, the coated wires disclosed in the reference moreover are not applicable to low temperature curable, let alone radiation curable topcoats, to which the present invention particularly pertains.
All in all, it is desired to provide a method of coating metal wires suitable for insertion into a body cavity, particularly guidewires, and more particularly coiled wires, which satisfies one or more of the following: allowing a good adhesion of the coating to the metal wire; having a good intercoat adhesion, providing a surface of desirable lubricity; having a sufficient elasticity to accommodate coiled wires; having a sufficient wear-resistance to be used for insertion into a medical instrument, and provide the desired performance on the basis of a coating layer that is capable (e.g. by being not too thin) to conform to the coiled wire. Desirably, the coating is elastic enough not to break when the coil is bent, and is not deformed after bending.
In particular, it is desired to provide a primer for use in coating a metal surface, particularly a metal wire, with a coating comprising a curable topcoat, the curing of which results in a lubricious, hydrophilic surface, which primer exhibits a favorable combination of flexibility, elasticity, adhesion to the metal surface and adhesion to the topcoat. Desirably this results in an eventual coating on the metal that has a good intercoat adhesion and water resistance. Systems satisfying these combined properties are as yet unknown.